There's no reason not to have sex when your partner has her period. It isn't an increased risk. It's just messy.
If you and your partner have both been tested for std's and don't have any, no risks just because of contact with her blood during her time of the month.
grace
If she has an std, then yes, there is a risk whether or not she is on her period. But if she does not have an std, then you are fine.
Any unprotected sex runs a risk of STD transfer, period or not.